One of the primary steps in the fabrication of modern semiconductor devices is the formation of a thin film on a semiconductor substrate by chemical reaction of gases. Such a deposition process is referred to as chemical vapor deposition (CVD). Conventional thermal CVD processes supply reactive gases to the substrate surface where heat-induced chemical reactions can take place to produce the desired film. Plasma CVD processes promote the excitation and/or dissociation of the reactant gases by the application of radio frequency (RF) energy to the reaction zone proximate the substrate surface thereby creating a plasma of highly reactive species. The high reactivity of the released species reduces the energy required for a chemical reaction to take place, and thus lowers the required temperature for such CVD processes.
In one design of plasma CVD chambers, the vacuum chamber is generally defined by a planar substrate support, acting as a cathode, along the bottom, a planar anode along the top, a relatively short sidewall extending upwardly from the bottom, and a dielectric dome connecting the sidewall with the top. Inductive coils are mounted about the dome and are connected to a source radio frequency (SRF) generator. The anode and the cathode are typically coupled to bias radio frequency (BRF) generators. Energy applied from the SRF generator to the inductive coils forms an inductively coupled plasma within the chamber. Such a chamber is referred to as a high density plasma CVD (HDP-CVD) chamber.
In some HDP-CVD chambers, it is typical to mount two or more sets of equally spaced gas distributors, such as nozzles, to the sidewall and extend into the region above the edge of the substrate support surface. The gas nozzles for each set are coupled to a common manifold for that set; the manifolds provide the gas nozzles with process gases. The composition of the gases introduced into the chamber depends primarily on the type of material to be formed on the substrate. For example, when a fluorosilicate glass (FSG) film is deposited within the chamber, the process gases may include, silane (SiH4), silicon tetrafluoride (SiF4), oxygen (O2) and argon (Ar). Sets of gas nozzles are commonly used because it is preferable to introduce some gases into the chamber separately from other gases, while other gases can be delivered to a common set of nozzles through a common manifold. For example, in the above FSG process it is preferable to introduce SiH4 separately from O2, while O2 and SiF4 can be readily delivered together. The nozzle tips have exits, typically orifices, positioned in a circumferential pattern spaced apart above the circumferential periphery of the substrate support and through which the process gases flow.
As device sizes become smaller and integration density increases, improvements in processing technology are necessary to meet semiconductor manufacturers"" process requirements. One parameter that is important in such processing is film deposition uniformity. To achieve a high film uniformity, among other things, it is necessary to accurately control the delivery of gases into the deposition chamber and across the wafer surface. Ideally, the ratio of gases (e.g., the ratio of O2 to (SiH4+SiF4)) introduced at various spots along the wafer surface should be the same.
FIG. 1 illustrates a typical undoped silicate glass (USG) deposition thickness variation plot 46 for a conventional deposition chamber such as the chamber described above. The average thickness is shown by base line 48. As can be seen by plot 46, there is a relatively steep increase in thickness at end points 50 and 52 of plot 46 corresponding to the periphery 42 of substrate 20. The center 54 of plot 46 also dips down substantially as well.
U.S. patent application Ser. No. 08/571,618 filed Dec. 13, 1995, the disclosure of which is incorporated by reference, discloses how plot 46 can be improved through the use of a center nozzle 56 coupled to a third gas source 58 through a third gas controller 60 and a third gas feed line 62. Center nozzle 56 has an orifice 64 positioned centrally above substrate support surface 16. Using center nozzle 56 permits the modification of USG deposition thickness variation plot 46 from that of FIG. 1 to exemplary plot 68 of FIG. 2. Exemplary deposition thickness variation plot 68 is flat enough so that the standard deviation of the deposition thickness can be about 1 to 2% of one sigma. This is achieved primarily by reducing the steep slope of the plot at end points 50, 52 and raising in the low point at center 54 of plot 46.
With the advent of multilevel metal technology in which three, four, or more layers of metal are formed on the semiconductors, another goal of semiconductor manufacturers is lowering the dielectric constant of insulating layers such as intermetal dielectric layers. Low dielectric constant films are particularly desirable for intermetal dielectric (IMD) layers to reduce the RC time delay of the interconnect metallization, to prevent cross-talk between the different levels of metallization, and to reduce device power consumption.
Many approaches to obtain lower dielectric constants have been proposed. One of the more promising solutions is the incorporation of fluorine or other halogen elements, such as chlorine or bromine, into a silicon oxide layer. It is believed that fluorine, the preferred halogen dopant for silicon oxide films, lowers the dielectric constant of the silicon oxide film because fluorine is an electronegative atom that decreases the polarizability of the overall SiOF network. Fluorine-doped silicon oxide films are also referred to as fluoro silicate glass (FSG) films.
From the above, it can be seen that it is desirable to produce oxide films having reduced dielectric constants such as FSG films. At the same time, it is also desirable to provide a method to accurately control the delivery of process gases to all points along the wafer""s surface to improve characteristics such as film uniformity. As previously discussed, one method employed to improve film deposition uniformity is described in U.S. patent application Ser. No. 08/571,618 discussed above. Despite this improvement, new techniques for accomplishing these and other related objectives are continuously being sought to keep pace with emerging technologies.
The present invention is directed toward an improved deposition chamber that incorporates an improved gas delivery system. The gas delivery system helps ensure that the proper ratio of process gases is uniformly delivered across a wafer""s surface. The present invention is also directed toward a method of depositing FSG films having a low dielectric constant and improved uniformity. This is achieved by a combination of (1) the uniform application of the gases (preferably silane, fluorine-supplying gases such as SiF4 or CF4, and oxygen-supplying gases such as O2 or N2O) to the substrate and (2) the selection of optimal flow rates for the gases, which preferably have been determined as a result of tests using the particular chamber. In some embodiments, the deposited FSG film has a dielectric constant as low as 3.4 or 3.3. Preferably, the dielectric constant of the FSG film is at least below 3.5.
The improved deposition chamber includes a housing defining a deposition chamber. A substrate support is housed within the deposition chamber. A first gas distributor has orifices or other exits opening into the deposition chamber in a circumferential pattern spaced apart from and generally overlying the circumferential periphery of the substrate support surface. A second gas distributor, preferably a center nozzle, is used and is positioned spaced apart from and above the substrate support surface, and a third gas distributor delivers an oxygen-supply gas (e.g., O2) to the chamber through the top of the housing in a region generally centrally above the substrate. This is preferably achieved by passing the oxygen through an annular orifice created between the center nozzle carrying the silane (and any other gases) and a hole in the top of the housing. In one embodiment the first gas distributor includes first and second sets of nozzles.
In one embodiment of the method of the present invention, an FSG film is deposited from a process gas that includes silane, oxygen and SiF4. Oxygen and SiF4 are delivered together to the chamber through the first set of nozzles, and silane (or silane and SiF4) is delivered through the second set of nozzles. Mixing the SiF4 with oxygen and introducing this combination through the first set of nozzles reduces equipment complexity so cost can be reduced. Silane (or silane and SiF4) is also injected into the vacuum chamber from the second gas distributor to improve the uniform application of the gases to the substrate over that which is achieved without the use of the second gas distributor, and oxygen is delivered through the third gas distributor. In this way, oxygen is provided both from the sides through the first set of nozzles of the first gas distributors, preferably mixed with SiF4, and also in the same region as silane above the substrate. Also, the passage of the oxygen through the annular orifice keeps reactive gases within the chamber from attacking the seals used between the top of the housing and the body from which the center nozzle extends. This advantage is retained if silane is passed through the annular orifice and oxygen through the center nozzle.
Film thickness and dielectric constant uniformity is also enhanced by ensuring that the temperature of the substrate remains uniform across the substrate and using a source RF generator designed to achieve sputtering uniformity.
One of the primary aspects of the method of the present invention is the recognition that it is very important to ensure the uniform distribution of oxygen entering the chamber. This is achieved by flowing oxygen both from the top of the chamber and from the sides of the chamber. Additionally, by the appropriate configuration of the oxygen flow path through the top of the chamber, the oxygen can serve to protect the sealing element from deleterious effects of coming in contact with reactive gases such as fluorine.
In addition to the need to supply the gases to the substrate uniformly, it is necessary to use the correct proportion of the gases, for example O2, SiH4 and SiF4, to deposit a stable film and achieve a minimum dielectric constant for that film. The proper flow rates for each will differ according to the particular chamber used. Accordingly, it is a further aspect of the invention to test a variety of flow rate proportions to discover which set of flow rates provides a high quality dielectric film with a minimum dielectric constant.